This invention relates to a transmitter for determining the speed and angular position of a rotating shaft, such as those employed in sewing technology, and more particularly, to such a transmitter employing a coded disc driven synchronously with a working shaft and having a series of circularly distributed light and dark areas, and a stator cooperating with same and including a luminous emitter and a photosensitive receiver.
In British Pat. No. 102653 a transmitter is disclosed in which the receiver supplies a signal in which the momentary amplitude depends on whether a light or dark area of the coding disc happens to be positioned in the path of the beam between the emitter and the receiver. This alternating voltage signal drives a discriminator which delivers a direct voltage signal which, for its part, is a function of the frequency of the alternating voltage signal of the receiver. The direct voltage signal thus obtained is compared to a direct datum voltage representing the nominal speed value. The clutch or brake of the clutch motor are actuated as a function of the differential signal generated on the basis of this comparison. The light-dark control of the receiver by the light and dark areas of the coding disc occurs perfectly in this known system only as long as the width of the receiver measured in the direction of displacement of the coding disc is equal to or smaller than the halving division of the coding disc, i.e., in the case of identical widths for the light and dark areas, equal to or smaller than the width of a light area or dark area. However, snce photosensitive receivers require an appreciable width of photosensitive surface in order to obtain a signal of adequate strength for further processing, the known system allows a relatively coarse graduation only. This means that the diameter of the coding disc becomes undesirably large or that the number of pulses obtained for each complete revolution of the coding disc becomes relatively small. If the receiver is wider than half the width of graduation, parts of the receiver surface are always situated opposite one or more dark areas and other parts of the receiver surface are always situated opposite one or more light areas, which reduces the signal amplitude. The effective signal of the receiver diminishes as the width of the receiver is increased in relation to the graduation of the coding disc. Moreover, the system has the disadvantage that the effective signal goes down to zero if the width of the receiver is equal to an integral multiple of the semi-graduation. This means that when a coding disc is replaced by a coding disc having a different graduation, the photosensitive receiver must also be replaced by a receiver of different width if appropriate, to obtain an effective signal.
In order to solve these problems, a transmitter of the above type should provide an effective signal allowing an unobjectionable evaluation even in the case of a coding disc having a small diameter and/or narrow coding disc graduation. It should be possible to make use of relatively wide and thus powerful photosensitive receivers for optionally narrowly spaced graduations of the coding disc. The effective signal obtained should be substantially unaffected by the ratio between the division of the coding disc and the width of the receiver.